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“I decided to create a 3-D printed, custom-scanned prosthetic arm for my dad, who is an amputee,” St. Louis high school student Robbie Frei told KTVI.

To get started, the Priory High School senior – who wants to go into robotics in college – designed a three-month thesis project. His mission: to build a 3-D printed arm for his father, who was injured in an RPG attack in 2003 in Iraq.

But first, he created an arm adapter for the Nintendo Switch.

“When my dad was injured in the Marines he wasn't able to play video games with us for about 10 years, and after I created that 3-D printed adapter he was able to play with us full speed,” Frei said.

Soon Frei was fine-tuning his designs, creating a custom scanned prosthetic, modeled after his dad`s left hand.

The high school senior mirrored the scan, creating a right-handed model that could throw a ball.

“He knows how to get projects done and comes up with amazing things,” said robotics advisor Derek Ward. “He's been on the robotics team for six years now. So, just to watch him go from a seventh-grader to a 12th-grader, (I'm) not surprised he can pull something like this off.”

After a lot of trial and error, Frei figured out a way to craft all the parts on the three-dimensional printer, constructing a design in which his dad controls the finger movements.

Frei is a National Merit finalist who credits mathematics for helping him understand a challenge and work towards a solution.

“As soon as I start to test different alternatives I start to narrow in on something,” Frei said. “It's sort of more discovery than inventing, in a way.”